江淮流域梅雨期持续性强降水及其10～30d低频环流特征

THE CHARACTERISTICS OF PERSISTENT HEAVY RAIN EVENTS AND 10～30 DAY LOW-FREQUENCY CIRCULATION IN YANGTZE-HUAIHE RIVER BASIN DURING MEIYU PERIOD

利用1961-2010年中国556站的逐日降水资料和NCEP/NCAR逐日再分析资料,采用合成分析等方法探讨了江淮流域梅雨期持续性强降水的低频振荡特征及其发生前后低频大气环流场的演变特征.结果表明,汀淮梅雨持续性强降水存在显著的10～ 30 d低频振荡特征.持续性强降水发生时,汀淮流域对流层高层受东海低频反气旋西北部的偏西气流控制,使得南亚高压位置偏东,加强了高层的辐散型流场;对流层中层,中高纬度地区存在“＋、-、＋”的低频位势高度中心,蒙古低频低压使得极地冷空气易于南侵;对流层低层,汀淮流域受低频P-J波列西段的台湾岛低频反气旋影响,并伴随强烈的对流活动,此反气旋使得西太平洋副热带高压向更西的位置伸展;因此低层辐合、高层辐散使得汀淮流域表现为强烈的低频上升运动,同时低频水汽从孟加拉湾-南海一带输送到汀淮流域,为持续性强降水的发生提供了有利的低频环流条件.另外,在持续性强降水发生前后,低层低频正涡度向北、向西传播,高层低频负涡度向南、向东传播,高低层斜压结构明显,共同作用于汀淮流域,维持持续性强降水过程.

The characteristics of persistent heavy rain events and 10～30 d low-frequency circulation in Yangtze-Huaihe river basin during Meiyu period are studied by using daily precipitation data from 756 stations in China and the NCEP/NCAR daily reanalysis data from 1961 to 2010.The results show that a 10～ 30 d period is obvious during persistent heavy rain events.When a persistent heavy rain event occurs,the rainfall in Yangtze-Huaihe Rivers basin is influenced by the westerly flow of a low-frequency anticyclone located over the East China Sea in the upper troposphere,which results in the eastward extension of South Asian High and the increase of divergence circulation.In the middle troposphere,there is an anomalous geopotential height center in middle and high latitude area.A Mongolian low-frequency low pressure makes polar cold air invade the basin.In the lower troposphere,the low-frequency anticyclone located over western of a Pacific-Japan wave train causes the western North Pacific subtropical high to extend westward,which affects the rainfall and strong convective activity in the basin.The convergence （divergence） circulation in the low （upper） level leads to significant upward movement in the basin.Meanwhile,abundant water vapor is transported from the Bay of Bengal and South China Sea to the basin.In addition,the northward and westward propagation of positive vorticity in the lower troposphere is coupled with the southward and eastward propagation of negative vorticity in the upper troposphere.Their structure is baroclinic in the basin during the persistent heavy rain events.